---
title: 'PRATUSH Radiometer: 21-cm Cosmic Dawn Probe'
url: https://www.emergentmind.com/topics/pratush-radiometer
type: topic
---

# PRATUSH Radiometer: 21-cm Cosmic Dawn Probe

PRATUSH, short for **Probing ReionizATion of the Universe using Signal from Hydrogen**, is a proposed space-based radiometer for detecting the **global, sky-averaged redshifted 21-cm signal** from the **Cosmic Dawn (CD)** and **Epoch of Reionization (EoR)**. The experiment concept is centered on precision low-frequency spectral radiometry in a radio-quiet orbital environment, with a broader science envelope of **40–200 MHz** and a **baseline design** optimized for the **55–110 MHz** Cosmic Dawn band. In its baseline form, PRATUSH combines a smooth wideband antenna, calibrated analog bandpass measurements, in-situ antenna impedance monitoring, and an FPGA-based digital correlation spectrometer, with lunar-farside operation providing the principal mitigation of terrestrial radio-frequency interference (RFI) [2507.05654].

## 1. Scientific target and observational regime

The primary scientific objective of PRATUSH is the detection of the **global 21-cm signal** from neutral hydrogen during CD/EoR. In this context, the 21-cm hyperfine line at rest frequency **1420.406 MHz**, when redshifted to meter wavelengths, traces the average thermal and ionization history of the intergalactic medium. The targeted cosmological signature is a weak spectral distortion with a **maximum amplitude below a few hundred mK**, whereas astrophysical foregrounds are **hundreds to thousands of kelvin**, so spectral smoothness and calibration fidelity are fundamental rather than auxiliary requirements [2507.05655].

The PRATUSH literature treats the science observable as the sky-averaged spectrum rather than an image-domain quantity. The experiment is intended to detect the global signal’s **turning points / inflections**, which encode the underlying astrophysics and cosmology. The mission overview also uses the controversial **EDGES low-band claim** as a benchmark, while the laboratory-model paper emphasizes that the instrument class is aimed at a high-confidence detection of the global 21-cm signal from **Cosmic Dawn** and the **Epoch of Reionization** [2507.05654].

Two mission phases are described in the laboratory-model digital-receiver paper: **Phase I: Low Earth Orbit (LEO)**, intended as a technology demonstration and pathfinder, and **Phase II: Lunar orbit / lunar farside operation**, identified as the main science phase because the lunar farside offers exceptional shielding from terrestrial and solar radio noise. The experiment-concept paper presents the **lunar-orbiter baseline** optimized for farside science [2507.05655].

## 2. Mission concept and baseline payload

PRATUSH was proposed to the **Indian Space Research Organization (ISRO)** during the announcement of opportunity for science payloads in **2018** and is in the **pre-project studies phase**. The concept model is under development and is expected to lead to the engineering model followed by the flight model, subject to mission approval. Because of the measurement’s sensitivity, PRATUSH is designed to operate as a **dedicated solo spacecraft** in order to avoid payload-to-payload interference [2507.05654].

The rationale for the lunar-farside configuration is both observational and instrumental. The farside strongly suppresses **terrestrial RFI**, including the terrestrial **FM band (88–108 MHz)** that overlaps the science band; it also avoids the terrestrial ionosphere, ground coupling, and horizon/topography-induced contamination that affect ground-based instruments. The concept of operation is to observe in the radio-quiet region on the lunar farside and downlink the data on the nearside [2507.05654].

The baseline payload is organized into three main subsystems:

| Subsystem | Function | Baseline implementation |
|---|---|---|
| Antenna | Smooth beam and smooth return loss | **Monocone over a shaped reflector** |
| RF system | Bandpass calibration, gain conditioning, in-situ antenna return-loss measurement | Analog receiver plus **one-port VNA** section |
| Digital receiver | Digitization, correlation spectroscopy, flagging, averaging, storage/downlink of reduced products | FPGA-based digital backend |

The baseline antenna is a **monocone with a shaped reflector**, optimized over **55–110 MHz** and fitted to a **2 × 12U bus**. The reflector shape is based on an optimized **log-spiral curve** to keep the return loss smooth. The electronics are placed in **EMI-shielded compartments**, and the antenna is mounted above the spacecraft bus. This configuration reflects the project’s central design requirement: broadband response is insufficient unless the antenna and receiver remain spectrally smooth enough that residual structure after calibration is nearly Gaussian and at or below the **mK** scale [2507.05654].

## 3. Radiometric measurement principle and statistical sensitivity

PRATUSH is designed around the idea that the measured low-frequency sky spectrum should be as smooth as possible except for the cosmological contribution. Foregrounds are expected to be spectrally smooth, whereas the 21-cm signal has turning points; accordingly, the instrument is required to deliver a **bandpass-calibrated, antenna-corrected spectrum** whose residuals after smooth foreground modeling are at the **mK level or below**. The mission concept uses **Maximally Smooth (MS)** functions as the diagnostic of spectral smoothness [2507.05654].

The antenna response enters directly through the return loss. The experiment concept defines the antenna temperature after return loss as

$$
T_A = T_{sky}(1-|\Gamma|^2),
$$

where $\Gamma$ is the antenna reflection coefficient. In the validation pipeline, simulated beam and return-loss curves are combined with **GMOSS**, lunar emission contributions, and candidate global 21-cm signal models. A specific result reported for the baseline concept is that **return-loss correction is essential**: without it, a smooth foreground-only spectrum can have residuals of order **7.1 mK**, whereas with in-situ return-loss correction the Cosmic Dawn signal becomes distinguishable [2507.05654].

A statistical foundation directly relevant to a PRATUSH-type radiometer is provided by the derivation of the radiometer equation in "Statistical Topics Concerning Radiometer Theory" [1507.04280]. In that treatment, the instantaneous amplitude is modeled as a zero-mean normal random variable,

$$
A = \sigma z,
$$

with measured power proportional to the square,

$$
A^2 = \sigma^2 z^2.
$$

Using the Nyquist/Johnson relation,

$$
P = kT\beta,
$$

and the time-bandwidth product,

$$
N = 2\beta\tau,
$$

the temperature sensitivity becomes

$$
\sigma_T = \frac{T}{\sqrt{\beta\tau}}.
$$

For a PRATUSH-like radiometer, this gives the ideal thermal-noise floor: sensitivity improves as $1/\sqrt{\tau}$ provided that the signal is Gaussian and white over the relevant band, the variance is finite and stable, and the samples entering the average are statistically independent [1507.04280].

The same statistical analysis also identifies the principal caveat for long integrations. Simulations of **$1/f$** and **$1/f^2$** noise show that these processes are non-stationary, their variance grows with observing time, and they produce long-range correlations. When small gain fluctuations of this form are added to an otherwise white-noise radiometer signal, the measured mean-power distribution broadens beyond the radiometer-equation prediction and the improvement with integration ceases to follow the ideal $1/\sqrt{\tau}$ law. This is the formal statistical basis for treating gain stability, calibration cadence, and low-frequency systematics as first-order performance drivers for PRATUSH rather than secondary engineering details [1507.04280].

## 4. Calibration architecture and suppression of instrumental structure

The baseline PRATUSH calibration scheme is inspired by **SARAS-3** and combines a **Dicke switch**, a **noise-source / attenuator calibration unit**, a **phase switch**, and a **double-differencing** method. The receiver cycles through six states, `OBS00`, `OBS11`, `CAL00`, `CAL01`, `CAL10`, and `CAL11`. The calibrated bandpass spectrum for a cycle is written as

$$
T_{bp} = T_{abs}\frac{(OBS00 - OBS11) - (CAL00 - CAL01)}{(CAL10 - CAL11) - (CAL00 - CAL01)},
$$

where $T_{abs}$ is an absolute temperature scale from hot/cold load calibration. In this formulation, differencing suppresses receiver offsets and internally generated noise while isolating the sky contribution from calibration artifacts [2507.05654].

A distinctive feature of PRATUSH is the inclusion of a dedicated **one-port VNA** section for in-orbit measurement of the antenna’s complex impedance or $S_{11}$. The VNA uses a **DAC-generated swept tone**, **open**, **short**, and **50 $\Omega$** standards, and an RF switch network, with calibration to correct cable, switch, and fixture effects. The motivation is explicit: the antenna may change because of **thermal cycling** across lunar day/night transitions and because of the **plasma environment** in lunar orbit, so the impedance model must be monitored in situ if residual systematics are to remain below mK-level contamination [2507.05654].

The concept study also treats internal spectral contamination as a design constraint. For standing waves in cables and internal reflections, the quoted periodicity estimate is

$$
\delta f = \frac{v_{light}}{2\times \sqrt{\epsilon_r}\times L}.
$$

The stated design intent is to keep cables only **centimeters long**, so the standing-wave period is much broader than the cosmological spectral structure and therefore less likely to masquerade as the signal. **EMI from spacecraft electronics** is identified as perhaps the largest technical risk, and the shielding requirement is described as likely needing effectiveness of **$\gtrsim 120$ dB**, supported by multi-layer shielding, linear power supplies, careful clock selection, twisted pairs/coax, PCB shielding practices, strong on-ground EMI testing, and maximal physical separation where possible [2507.05654].

A common misconception is that lunar-farside radio quietness alone would make global-signal detection straightforward. The PRATUSH design literature instead treats radio quietness, antenna smoothness, calibration accuracy, standing-wave control, and EMI suppression as coupled requirements. This suggests that the instrument’s scientific performance depends as much on suppressing internally generated spectral structure as on external RFI avoidance.

## 5. Digital correlation spectrometer and laboratory implementation

The **Digital Correlation Spectrometer (DCS)** is a central subsystem of PRATUSH. Its functional roles include **analog-to-digital conversion**, **phase switching**, **spectral channelization**, **self- and cross-power spectra generation**, **RFI mitigation**, **calibration control**, **data packetization and transfer**, and **onboard recording / pre-processing**. In the laboratory model, the two analog receiver outputs are split into **0° and 180° phase-shifted paths**; this phase-switching arrangement is used to help cancel additive system contributions from the analog chain and samplers. The digital pipeline then performs digitization, **windowing**, **FFT-based channelization** in an **FX correlator** architecture, spectral integration on the FPGA, and Ethernet streaming of the integrated spectra to the controller [2507.05655].

The flight-baseline concept and the laboratory model are related but not identical:

| Aspect | Baseline concept | Laboratory model |
|---|---|---|
| ADCs | **Two 12-bit ADCs**, **250 MSps** | **Two quad 10-bit ADCs** (**EV10AQ190CTPY**), **250 Msps** |
| FPGA / channelization | **Virtex-5QV FPGA**, **2048-point FFT**, **244 kHz** resolution | **Virtex-6 FPGA** (**XC6VSX315T-FF1516-2**), **16384-channel spectrum**, **30.516 kHz** effective resolution |
| Control / integration | **16384 spectra averaged** onboard over about **134 ms** | **On-chip integration time ≈ 134 ms**; **Raspberry Pi 4 Model B** as master controller |

In the laboratory implementation, the digital receiver is built around the **pSPEC** platform. The ADC input full-scale range is about **500 mVpp** (approximately **-2 dBm**), and the ADC inputs are maintained at about **-27 dBm total power** to leave roughly **4 bits of headroom** against strong RFI and clipping. A **4-term Nuttall window** is applied before the FFT; the paper reports ideal sidelobe suppression of about **98 dB** and practical suppression of about **80 dB** with 10-bit ADCs and finite FPGA precision. The firmware uses a **16384-point FFT** implemented as a **split-FFT $M\times N$ architecture**, with two parallel **8192-point streaming FFT IP cores** plus a custom **2-point parallel FFT stage** [2507.05655].

The controller for the laboratory model is a **Raspberry Pi 4 Model B** SBC, selected for its compromise among compute capability, memory, I/O, Linux support, documentation, low power, and portability. The paper also emphasizes the engineering cost of this choice: lower CPU performance, limited memory and storage bandwidth, slower SD-card I/O, greater sensitivity to timing jitter, more difficult real-time coordination between acquisition and control, higher risk of data corruption during long runs, and the need to manage RFI from the SBC electronics. These issues are not incidental; the reported output data rate is about **8 MBps** (approximately **64 Mbps**), and long runs can show **3–10%** data corruption from packet drops [2507.05655].

A major contribution of the laboratory-model study is **dynamic flagging** for corrupted spectra. For channel $i$, the **median absolute deviation** is defined as

$$
MAD_i = median\left(|x_{ij} - \overline{x_i}|\right),
$$

where $x_{ij}$ is the value of channel $i$ in spectrum $j$ and $\overline{x_i}$ is the median value of channel $i$ across the dataset. This is converted to an approximate standard deviation through

$$
\sigma_i = k\cdot MAD_i,\qquad k \approx 1.4826,
$$

for Gaussian-distributed data. Channels exceeding a threshold in units of $\sigma_i$ are flagged, and if **16 consecutive channels** are flagged the entire spectrum is treated as corrupted and dropped. A **moving window** can also be used so that slow drifts do not cause over-flagging [2507.05655].

## 6. Demonstrated performance and related calibration developments

The laboratory digital receiver has been integrated with the PRATUSH laboratory-model analog receiver and tested end-to-end using **open**, **short**, **50 $\Omega$ load**, and **antenna simulator / shaped termination** states. The reported demonstrations include successful **bandpass calibration**, successful **state switching** over the six calibration states, stable acquisition and packet transfer from FPGA to SBC, generation of calibrated spectra after flagging and averaging, and validation that the receiver output is dominated by **thermal noise** at the expected level [2507.05655].

For an **8-hour** run with a **50 $\Omega$ termination**, the residuals after a maximally smooth fit had **rms ≈ 72 mK**. When the data were combined over **44 hours of effective integration**, the residuals reached **12.5 mK rms** at the native **30.51 kHz** resolution, and after boxcar averaging to **610 kHz** the residuals improved to **3.4 mK rms**. The residuals were reported to be **Gaussian distributed**, indicating no strong unmodeled spectral structure in the receiver response at the achieved level [2507.05655].

At mission scale, the concept study assumes a **2-year** lifetime, about **200 hours total** of usable prime-cone observing time, roughly **15% of orbital time** as useful science time, and an orbital period of about **2 hours**. The observing sequence is to perform a VNA measurement of antenna return loss, make the bandpass-calibrated sky observation, perform another VNA measurement before exiting the prime cone, and downlink the data when Earth is in view. Under these assumptions, the final noise level is stated to reach roughly **mK thermal-noise rms at 244 kHz resolution**. For sensitivity tests, the injected fiducial 21-cm absorption is modeled as a Gaussian with amplitude **$-180$ mK**, center frequency **78 MHz**, and **FWHM 23.5 MHz**; the posterior on the Gaussian parameters is reported as consistent with the injected signal [2507.05654].

A related calibration development of direct relevance to PRATUSH-style experiments is the physics-informed neural calibration framework demonstrated on the **REACH** receiver in "Radiometer Calibration using Machine Learning" [2504.16791]. That work addresses the same generic problem faced by global 21-cm radiometers: impedance mismatch, standing waves, spectral ripples, and receiver-chain systematics in a wide-beam, non-steerable instrument. The framework uses a **multi-layer perceptron** to infer source-independent **Noise Parameters** and **gain** from thermocouple data, PSD measurements, and VNA reflection-coefficient measurements, then propagates those parameters through an explicit receiver model. Reported performance includes **RMSE = 0.05 K** on relative residuals at **1 MHz** channel width over **60–130 MHz** on real receiver data, and **RMSE = 0.14 K** at **12 kHz** and **0.04 K** when binned to **1 MHz** in full-chain simulations. Although this is not presented as the baseline PRATUSH calibration chain, it is explicitly relevant to **PRATUSH-style** sky-averaged 21-cm radiometry, particularly where time-varying receiver behavior and mismatch-driven spectral structure limit conventional calibration assumptions [2504.16791].

Source: https://www.emergentmind.com/topics/pratush-radiometer